1. Field of the Invention
The present invention relates in general to the field of welding, and in particular to a new and useful process for conducting PAW on thicker steel stock than was previously possible.
2. Description of the Related Art
Plasma Arc Welding (PAW) is an arc welding process that produces coalescence of metal by heating with a constricted arc between an electrode and the workpiece (transferred arc) or between the electrode and the constricting nozzle (nontransferred arc). Shielding is generally obtained from the hot, ionized gas issuing from the torch. This plasma gas is usually supplemented by an auxiliary source of shielding gas. Shielding gas may be a single inert gas or a mixture of inert gases. Pressure is not applied, and filler metal may or may not be added.
Plasma arc welding, like gas tungsten arc welding (GTAW), uses a nonconsumable electrode. The PAW torch has a nozzle that creates a gas chamber surrounding the electrode. The arc heats the gas fed into the chamber to a temperature where it becomes ionized and conducts electricity. This ionized gas is defined as plasma. Plasma issues from the nozzle orifice at a temperature of about 30,000.degree. F. (16,700.degree. C.). See R. L. O'Brien, Welding Handbook --Welding Processes, Volume 2, 8th Edition, by the American Welding Society, Chapter 10--Plasma Arc Welding.
Two types of plasma arc welds can be made; melt-in welds and keyhole welds. The melt-in plasma arc welding process has a lower power density and produces a weld pool similar to the gas tungsten arc welding process. The keyhole plasma arc welding process is considered a high power density welding process since the arc has the capacity to penetrate through a thickness of material and make welds autogenously. The power density of the plasma arc welding process is controlled by the plasma gas flow rate where the higher the flow rate the higher the power density. However, at too high a power density and plasma gas flow rate the process will cut the material into two pieces rather than welding it into one piece.
The main advantages of keyhole PAW over the melt-in PAW or GTAW process is that the greater penetration capabilities of the keyhole PAW process reduces the number of weld passes required to fill the joint. In many cases, the weld can be completed in a single pass. In steels, the maximum thickness which can be welded with the keyhole PAW process on a square groove butt joint is reported to be 0.25 inch for single pass welds. See W. Lucas, TIG and Plasma Welding, Abington Publishing, England, Chapters 8 and 9, 1990. As the thickness of the joint increases over 0.25 inch, a multi-pass weld procedure is used where a groove, either a V or U, is machined into the base material with the point member butt land of thickness of no more than 0.25 inch. In multi-pass welds, the keyhole PAW process is used to weld the root pass and the melt-in PAW process is used to fill the joint groove.
In steels at thicknesses greater than 0.25 inch, the keyhole PAW process has not been used to weld the entire thickness in one pass because the catastrophic breakdown of the keyhole weld pool results in either partial plate penetration, excessive porosity, cutting and/or disfigured welds. At these thicknesses steady state welding with the keyhole PAW processes is quite difficult. Even more difficult is the control of transient conditions of the keyhole PAW process; i.e., the initiation and termination of the keyhole PAW mode. U.S. Pat. No. 4,885,455 discloses a process for terminating closed through-hole plasma arc welding.